Hybrid electric vehicles (HEVs) include both an engine and at least one electric motor to provide drive torque to the wheels. There are various types of HEVs. For example, “parallel” hybrid vehicles typically include clutches that selectively enable either or both the engine and the motor to provide drive torque. A “series” hybrid vehicle typically includes an electric motor that is always driveably connected to the road wheels, and an engine that is not mechanically connected to the wheels. In other words, the engine does not provide any of the torque required to propel the vehicle. Rather, in a “series” hybrid vehicle, the engine powers a generator to produce electrical energy that is stored in the battery and/or used by the motor.
One particular type of parallel hybrid vehicle includes an engine and a motor separated along a driveshaft by a clutch. The motor can operate (either alone or in combination with the engine) to provide positive drive torque to the wheels. The motor can also act as a generator and provide negative torque when converting mechanical energy from the driveshaft into mechanical energy to be stored in a battery. This negative torque can be applied when, for example, the engine is running and the clutch is at least partially engaged to connect the engine to the motor. During changes in operational mode of the motor, forces on the clutch can vary drastically and quickly.